Hello wei-long,
This page can help out with providing available steps for AirPlaying video to an Apple TV: Use AirPlay to stream audio and video on Apple TV - Apple Support
Do one of the following:
* Mirror the screen of an iOS or iPadOS device on Apple TV: On the iOS or iPadOS device, swipe to open Control Center, tap Screen Mirroring
, then select Apple TV in the list of AirPlay devices.
* To end mirroring, make sure Control Center is open on your device, tap
, then tap Stop Mirroring.
* Stream video from an iOS or iPadOS device to Apple TV: Start playing video on the iOS or iPadOS device, tap
in the video window, then select Apple TV in the list of AirPlay devices.
* To stop streaming, tap
at the top of the screen, then select Play on [iOS or iPadOS device].
* Note: If you have an Apple TV 4K and an iOS or iPadOS device that supports 4K HDR video, your videos from the Photos app will stream in full 4K HDR resolution.
1. Open Settings
on Apple TV.
2. Select AirPlay and HomeKit, then do any of the following:
* Turn AirPlay on or off: Select AirPlay to turn it on; select it again to turn it off.
* Allow access to Apple TV: Select Allow Access, then see “Set who can use AirPlay to stream content to Apple TV” below.
* Turn Conference Room Display on or off: See Stream to a conference room display.
* Fix a cropped AirPlay display: If your AirPlay display appears cropped on Apple TV, turn on AirPlay Display Underscan to show the entire image. If you see black bars around the image, turn this setting off.
* Select a Room: Select Room, then choose the room where you keep your Apple TV. For more information, see Set up Apple TV to stream audio to AirPlay 2–enabled devices.
* Turn doorbell notifications on or off: Doorbell cameras you’ve connected with HomeKit appear in a list under Notifications. Select a camera to turn it off; select it again to turn it on.
* For more information, see Switch users in the Profiles tab on Apple TV.
If it seems like only one app is unable to utilize AirPlay, and others are working well, then you may also try reaching out to the developer of the affected app. We were able to help locate the YouTube help site here: YouTube Help
Similarly, this page will have steps from Apple on how to contact an app's developer: How to contact an app developer - Apple Support
Cheers.